This invention relates to radiation hardened (xe2x80x9cradhardxe2x80x9d) devices which have improved resistance to damage by large (megarad) doses of ionizing radiation, or by single or plural high energy charged particles (xe2x80x9cSEExe2x80x9d particles).
Radiation hardened power MOSFETs and other MOSgated devices for use in space or other high radiation ambients have conflicting design requirements for resisting damage due to high doses of ionizing radiation on the one hand, and damage due to even single event high energy charged particles (xe2x80x9cSEExe2x80x9d) on the other hand. Thus, a thin gate oxide is desirable to resist high radiation (megarad) environments, while a relatively thick gate oxide is desirable to resist SEE effects.
More specifically, it is known that after exposure to a large total dose of ionizing radiation a positive charge will build up in the gate oxide to change the device threshold voltage. Further, there is an increase of interface traps at the silicon/gate oxide boundary. Both of these effects are reduced by using a thinner gate oxide, for example, one having a thickness of less than about 900 xc3x85.
Devices in a high radiation environment, for example, outer space, are also subject to damage or failure if struck by even a single high energy charged particle. Such charged particles which pass into or through the silicon generate a large number of electron-hole pairs in the depletion region of the device. Some of these charges collect on the gate oxide, resulting in a high potential across the gate oxide. Thus, a thicker gate oxide, for example, one thicker than about 1300 xc3x85 is desired to resist SEE failure.
Because of these diverse requirements, different manufacturing processes are used for a xe2x80x9cmegaradxe2x80x9d product, designed for use in a high total radiation dose environment, and an SEE product which is optimized for single particle effects.
In presently designed vertical conduction, multi-cellular MOSFET products, the charge collection at the oxide interface is in the drift region between cells. The device voltage is set in the charge in the inversion region. Thus, a design trade-off is necessary to set the gate oxide thickness for either a thin gate oxide for good total dose resistance or relatively thicker gate oxide for good SEE resistance.
In accordance with the present invention a MOSgated device (a power MOSFET, IGBT, GTO or other device employing a MOS gate) which has optimal oxide thicknesses for both total radiation dose resistance and SEE resistance is provided, using a trench design device. Thus, a known vertical conduction trench device has an invertible channel region on the sides of each trench, while the drift region lies along and under the bottoms of the trenches. Consequently, the gate oxide thickness at the walls of each trench can be relatively thin, and less than about 900 xc3x85 (preferably about 500 xc3x85) for optimal total dose resistance, while the bottoms of the trenches have a relatively thick oxide liner, for example, greater than about 1300 xc3x85 (and preferably about 3000 xc3x85) for optimal resistance to breakdown by single event effects.